The present invention relates to cellular communication systems, and more particularly to cell selection in a cellular communication system.
Cellular communication systems typically comprise a land-based network that provides wireless coverage to mobile terminals that can continue to receive service while moving around within the network's coverage area. The term “cellular” derives from the fact that the entire coverage area is divided up into so-called “cells”, each of which is typically served by a particular radio transceiver station (or equivalent) associated with the land-based network. Such transceiver stations are often referred to as “base stations”. As the mobile device moves from one cell to another, the network hands over responsibility for serving the mobile device from the presently-serving cell to the “new” cell. In this way, the user of the mobile device experiences continuity of service without having to reestablish a connection to the network. FIG. 1 illustrates a cellular communication system providing a system coverage area 101 by means of a plurality of cells 103.
In a telecommunication device such as a mobile phone/user equipment (UE) incorporating technology such as that which is in compliance with any known standards, such as the Global System for Mobile communication (GSM), Wideband Code Division Multiple Access (WCDMA), and/or the Third Generation Partnership Project's (3GPP's) Long Term Evolution (LTE), there is a need for a very precise time reference in order to be able to communicate with a base station. For this purpose, it is common to use a crystal oscillator-based (XO) circuit that provides both the UE demodulation frequency (e.g., carrier frequency) and sample rate reference. The term “crystal oscillator” is used generically throughout this document to refer to any type of crystal oscillator such as, but not limited to temperature-compensated crystal oscillators (TCXOs) and voltage-controlled crystal oscillators (VCXOs).
At various times, the generated UE demodulation frequency may be inaccurate by as much as ±17 ppm after factory calibration. This inaccuracy may be due to any or a combination of factors such as ambient temperature, aging, and inherent properties of the particular circuit or the calibration process in question. This means, for example, that when a UE tunes in to a carrier on the 2.6 GHz band, it may experience a frequency error of the received signal as large as ±45 kHz. This is too much for any message to be successfully received and decoded; generally the frequency error is required to be within plus/minus a couple of hundred Hz to allow for proper decoding of messages.
When a UE is connected to/camping on a cell, it keeps the frequency error within the range for which messages can be successfully decoded by repeatedly tuning its demodulation frequency to the carrier frequency being used by the base station. This procedure is referred to as automatic frequency correction (AFC), and is generally based on some or all of the channels below for each respective radio access technology (RAT):                GSM: Frequency Correction Channel (FCCH)        WCDMA: Synchronization Channel (SCH) and Common Pilot Channel (CPICH)        LTE: Synchronization Signal (SSIG) andCommon Reference Signal (CRS)        
Were the UE to stop correcting the demodulation frequency, after some while the frequency error would increase, thereby making it no longer possible to receive and successfully decode messages.
A more extensive approach is needed for the special case of the UE now being in an operational state but not having been able to tune its demodulation frequency to a cell belonging to the radio access network (e.g., when the UE is powered on after having been turned off, when the UE returns to a normal communication mode of operation after having been in a special flight-mode, or when the UE has been out-of-coverage for a while). The approach generally includes hypothesizing several frequency errors combined with so-called “cell search”, and is described, for example, in the background section of Axmon et al.'s U.S. Patent Publication No. 20110103534, entitled “Frequency Synchronization Methods and Apparatus” (hereinafter “Axmon et al. document”), which was published on May 5, 2011 and which is hereby incorporated herein by reference in its entirety. This particular combination of cell search and initial AFC is sometimes referred to as “initial cell search”, and is characterized by its being able to handle and identify large frequency offsets at the expense of longer processing time and/or radio time than is required for an ordinary cell search, which assumes a well-tuned UE demodulation frequency.
When the UE is searching for a first cell, it has to carry out initial cell search in order to account for frequency offsets. In case it finds a suitable or acceptable cell (herein generically referred to as a cell that is “qualified for camping”), it can camp on that cell and thereby maintain its UE demodulation frequency synchronized to the network. When it then continues to search for other cells, it can do so using the more efficient cell search technique because it does not need to account for a large demodulation frequency error.
The inventors of the subject matter described herein have recognized that existing cell searching techniques have problems. For example, if the UE is performing an ordinary cell search (i.e., as a result of the cell search being initiated when the UE's demodulation frequency synchronized to the network within acceptable limits) and encounters a cell that is unqualified for camping (e.g., a cell that the UE is not allowed to camp on for one or more reasons) then the UE has to continue searching for a qualified camping cell (e.g., suitable or acceptable). After some while, the UE will no longer be synchronized to the network, and will then have to use the initial cell search strategy when continuing the search on other carriers/bands/RATs.
As explained above, initial cell search requires substantially more time and UE resources than an “ordinary” cell search. Consequently, the more often initial cell search needs to be run, the longer it will take the UE to find a proper cell. This will have an impact on user experience and/or power consumption in some scenarios, particularly for multi-mode UEs supporting several frequency bands.
It is therefore desired to have improved cell searching strategies and apparatuses.